Mineral wool as insulation, outsulation, drainboard, rainscreen?
I am building a multi use, residential, building.
It will be built into the side of a hill, with a south exposure walk out basement.
A main floor with 12' walls, and a stick framed roof to provide for a second floor in the future.
It may eventually become a living quarters, but for now, is just a shop with no loft.
It is located in south central Ontario, I believe is climate zone 5 or 6.
We have cold winters, and humid summers.
The building will be heated in winter, and air conditioned in summer.
There will be a full basement. The main floor will be constructed of poured concrete on steel pan.
Both the basement, and main floor will each have in floor hydronic heating.
The walls are to be 2x6, the rafters will be stick framed 2x12.
The foundation is poured.
The roof will be steel sheet, and the siding reverse board and batten.
As the building will not be used on a daily basis, but I will need to keep it from freezing in the winter, I will have heat on all season. Cooling would only be done if the building is eventually used for habitation. But I would like to plan for that to happen, just encase it does....
Heat will come from hot water solar panels, and the rest of the south face of the roof will be covered with solar voltaic panels. I am hoping that I can insulate the building sufficiently to not have to use oil as backup heat.
I have been reading on here for a couple years now, about apples and turds, mineral wool in the attic, and many other very informative articles on building theories and designs, trussed walls, persist and remote, and eifs... However, for as many pros to each, there are just as many cons, and some very dedicated followers of each.
Perhaps I have my own biases now, but would like to stick to a few principles I think I have correct for my building climate zone, and intended use.
a. Insulation works better on the outside. Outside of the foundation wall, wall studs, and rafters. But it just makes sence to insulate those cavities, its cheap, and easy. SO both the cavity should be filled, and the exterior side of the wall covered with insulation as a thermal break.
b. Vapour barrier needs to be on the inside of the insulation. That may be on the out side of the studs if they are not filled with insulation. Although more difficult to do correctly considering all the penetrations such as electrical boxes, plumbing, lighting, etc. A vapour barrier just behind gypsum drywall is doable, and is code...
c. Roofs should be cold and ventilated. Using steel, or shingles, the roofing material should be kept cool with soffit to ridge ventilation.
SO, if I have these correct, here is what I would like to do.
- Coat with water resistant barrier, paint on or stick on.
- 2" of Roxul drainboard = R8.6 from footing up to top of Main floor sill plate. Approx 24" above grade..
- cement board fastened to 2x10 pressure treated sill plate to protect roxul drainboard from grade to siding, eventually covered with veneer stone, or stucco.. 2x10 sill plate will overhang foundation wall by 3". The 2x6 wall will sit flush with inside edge of sill plate.Flashing put on top of 3" overhanging sill plate will stick out far enough to cover top of veneer stone as well.
Main Walls: From Inside to out.
- 1/2" gypsum drywall
- 6 mil Poly vapour barrier
- 2x6 stud wall with R-22 mineral wool batt insulation between studs
- 2" of drainboard, or cavityrock mineral wool board insulation on exterior side of stud wall.
- Tar paper or Tyvek
- 2x4 horizontal strapping screwed through Roxul into studs.
- 1" thick pine board reverse board and batten would equal 2" thick siding attached to strapping.
- 1/2" gypsum drywall
- 6 mil poly
- 2x12 rafters on 16" center (22' length) with R-44 mineral wool batt insulation between rafters
- 1" R-4 mineral wool board on top of rafters, or on bottom of rafters
- 2x4 horizontal strapping
- high rib steel. or standing seam steel roof.
That is my proposal.
My questions are:
1. Are there any flaws with this proposal as far as where the vapour barrier is? I was worried about summer time cooling, and getting condensation on the insulation side of the 6mil poly?
2. Is the extra 1" of insulation required or useful on top or on bottom of the roof rafters?
3. Is there enough air flow under a high rib steel roof, or between the top of the 2x12 filled with 11" of insulation and the 2x4 strapping for adequate soffit to ridge ventilation?
4. My main wall assembly has an overhang, past the foundation, of 4.5" to the outside of the battens, the board would add another 1" thickness to the wall. That would make it possible to increase the foundation insulation thickness to 3" R12.9 and you would only see the sill flashing between the boards, in the reverse board and batten siding. Would it be worth it, to add another layer of drainboard to the foundation? My cost for that is only about $1000 more for the extra inch.
5. Is it possible to put 2x4 strapping over mineral wool board insulation that is 2" thick, and still attach reverse board and batten siding to the studs safely?
6. The major problem is the sill plate has no thermal break, or insulation. I do not know how to insulate that area, and still provide a good attachment point for the cement board.
Sorry for the long post, but wanted to make sure I posted all the details.
I am digging the hole for the foundation the first week of June, so I need to figure this out soon.
As you can see, I do not want to use foam, anywhere I dont have to. I may use some spray foam under the sill flashing to fill the void it would leave so water drains away from the wall, but dont see needing it anywhere else.
By not using any foam, I have a vapour open assembly everywhere to the exterior. That I think should make the summer time cooling ok if some condensation happens on the insulation side of the poly vapour barrier.
What do you think?
Thanks for any advise.
Posted May 28, 2011 7:08 PM ET
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